The purpose of this study is to investigate the control of insulin and glucagon release in the fetus and neonate. A pancreatic glucagon radioimmunoassay has been developed. In vivo studies have investigated the effects of different experimental models for intrauterine growth retardation in the rat upon hormonal response to fasting. Intrauterine growth retardation has been induced by several different types of maternal stress, including sham surgery, and forced maternal exercise. In addition, the isolated rat pancreatic islet has been used as a model system. Islets and pancreatic pieces from animals of different ages, from fetus to adult, are incubated in the presence of substances known to stimulate the release of insulin and glucagon in the adult animal. Induction of a mature pattern of insulin and glucagon response will be attempted by chronic exposure of the fetus and neonate to these various stimuli. It is expected that pancreatic islets of fetal and neonatal animals of other species, including primates, may be studied. Information derived from this investigation should shed light on the nature of carbohydrate homeostasis in the newborn specifically and on the nature of control of pancreatic endocrine function in general.